Triple Oxygen and Clumped Isotope Compositions of CO2 in the Middle Troposphere

We report the oxygen isotopic anomaly (Delta O-17) and clumped isotopic composition (Delta(47)) of CO2 sampled from the middle troposphere (similar to 10 km above ground) during two CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flights in summer and winter seasons: one from Dusseldorf to Isla Margarita on 11 August 2001 and the other from Puerto Plata to Munich on 21 February 2002. The mid-tropospheric Delta O-17 values were higher than the near-surface values by 0.051 +/- 0.010 parts per thousand, indicating enhanced stratospheric influence. The Delta(47) values were also higher by 0.160 +/- 0.015 parts per thousand compared to that from the lower troposphere. Stratospheric influence is supported by the observed correlations of the Delta O-17 values with the mixing ratios of N2O, O-3, and (CO)-C-14, which carry stratospheric signatures. The increase in the anomaly in the free troposphere represents a balance between the stratosphere-troposphere exchange fluxes and biogeochemical/hydrospheric isotopic resetting rates at the Earth surface. Using a simple two box model with the free tropospheric Delta O-17 values, an average surface exchange rate of CO2 between 327 and 772 PgC/year is inferred, giving a CO2 turnover time of 1.4 to 2.8 years in the atmosphere. The present estimates are based on a rather small set of free tropospheric data and have relatively large uncertainties, but they agree with the existing model and proxy-based estimates.